1. Field of the Invention
The present invention relates to an improved process for making steel in an electric arc furnace (EAF), which uses molten high carbon ferrous metal as a part of the EAF charge mix. The invention particularly relates to an improved process for steel making in an EAF and for making molten high carbon ferrous metal, either in a cupola or in a blast furnace or in other units which are used for making high carbon ferrous metal.
2. Description of the Related Art
One can classify the EAF steel making processes used in the prior art into two types: referred to hereinafter as Type I and Type II. In the Type I EAF steel making process, only solid charge is used such as: steel scrap (scrap): hot briquetted iron (HBI): direct reduced iron (DRI) which could be sponge, pellets, etc. and in some instances, solid pig iron or pig iron jam-as the EAF""s metallic raw material. Such an EAF process and operation, incorporate the usual complements of the currently known developments in the EAF process technology, including the foamy slag practice. Type I is the traditional EAF steel making process.
The Type II EAF steel making process is a relatively new development. This process is replacing the Type I process. The Type II process uses a liquid charge which is either molten hot metal obtained from a blast furnace or a COREX furnace or molten cast iron metal from a cupola (all the molten high carbon ferrous metals obtained from these three or other similar sources, are hereinafter referred to as xe2x80x98hot metalxe2x80x99xe2x80x94as one part of the raw material. Whereas the other part of the raw material consists of a solid charge, such as scrap; DRI; HBI; etc. In the Type II EAF process, a foamy slag is produced and maintained by the addition of oxygen and carburiser, as is done in the Type I process.
When compared to the steel making process of Type I, the use of the Type II EAF steel making process results in increased productivity and decreased power consumption in the EAF.
However, the steel making process of Type II also has following five major limitations when assessed all by itself.
First, it uses a considerable amount of gaseous oxygen to remove carbon, silicon and manganese from the molten hot metal and also to maintain a high oxygen potential to achieve phosphorous removal. The addition of a large quantity of gaseous oxygen, in turn, requires a large amount of carburiser so that a foamy slag can then be produced.
Second, the process uses a hot metal of conventional chemistry, that is, with carbon higher than 3%, with silicon usually higher than 0.4% and with manganese usually higher than 0.3%. Such a hot metal is usually prepared in a cupola or in a BF or in a similar unit. But the removal of higher levels of carbon, silicon and manganese requires additional oxygen, energy, time, fluxes and other consumables, during the steel making in the EAF. This is not only a wastage of the metalloids and alloying elements, such as carbon, silicon and manganese, but results in needless waste generation and corresponding loss of iron into the slag as iron oxide. But even more significant is the fact that the presence of higher levels of silicon and manganese dissolved in the molten ferrous metal delays the onset of carbon boil. This results in slower heat transfer from the electric arc to the metal to be melted. This delays melting and consumes additional energy.
Thus, in this process, the solid charge, such as, scrap, HBI, DRI, etc., is first melted after which the hot metal is poured into the EAF. This requires that an arc be produced to generate the heat required to melt the solid charge. This, in turn, results not only in higher electrode consumption but also prevents accelerated heat transfer from the arc to the solid charge as required for melting. Because, a major part of the heat transfer to the metallic charge in such instances occurs by radiation and conduction, at first from the arc to the slag layer and subsequently only by conduction from the slag to the metallic charge after the charge is partly melted and the slag forms. The slag is a poor conductor of heat and so is the solid metallic charge when it is added inside the EAF hearth since the charge gets automatically piled up with a considerable amount of void space in it. Therefore, the heat transfer, both through the slag and through the piled up metallic charge is relatively slow, resulting in a poor utilization of electrical energy to generate heat for melting the solid charge.
Fourth, DRI, if used, has to be of a high quality, that is, high in metallic iron and low in oxygen which is present in the DRI as iron oxide. Such a DRI is more highly priced.
Finally, the large quantity of silicon and manganese in the hot metal and higher oxygen potential used produce larger volumes of the oxides of silicon, manganese and iron. This in turn produces a larger slag volume requiring a larger quantity of flux to maintain a given level of basicity and a correspondingly larger loss of iron as oxide into the slag.
The main object of the present invention is to propose an improved process for EAF steel making without the use of gaseous oxygen and under an oxygen potential which is relatively low, so that the losses of iron and alloying elements are minimal.
Yet another object of the present invention is to propose an improved process for EAF steel making which uses resistive heating of the slag phase so that, consequently, the harmonics and resonance generated in the electrical circuit during arcing are significantly reduced, the sophisticated electrical circuitry currently used to maintain a stable arc is avoided, the power factor improves and electrical energy consumption decreases, the electrode consumption is less, electrode breakage is less, heat is generated mainly in the slag phase, and heat transfer is more effective producing an accelerated heating-melting-refining and, consequently, a decreased power consumption.
A further object of the present invention is to propose an improved process for EAF steel making with significantly decreased consumption of power, fluxes and other consumables.
Still a further object of the present invention is to propose an improved process of EAF steel making with the sequence of charging and the proportion of the charge mix (which consists of molten high carbon ferrous metal, scrap, HBI and DRI) in the EAF, so arranged and selected and also the chemistry of hot metal so adjusted, that the carbon boil starts as soon as the solid charge starts melting and the carbon boil continues just as long as the melting-refining continues, and that the carbon boil occurs and produces turbulence in the entire volume of molten metal bath, instead of being localized only at the slag/metal interface, and thereby producing very efficient heat transfer between the source of heat in the EAF and the entire molten metal bath, and also accelerates carbon removal.
Yet a further object of the present invention is to propose an improved process of EAF steel making with the slag basicity at a level higher than that conventionally used in most EAF steel making processes, but without significant increase in the consumption of flux.
Yet another object of the present invention is to produce from a cupola, the molten high carbon ferrous metal of a chemistry which not only facilitates steel making in the EAF but also facilitates the production of this metal from a cupola (or even from a blast furnace or any other furnace which produce such metal) and requires less expensive raw material, no ferroalloy addition and a lower consumption of coke.
According to this invention, there is provided an improved process for making steel in an electric arc furnace (EAF) using molten high carbon ferrous metal as a part of the EAF charge mix, which comprises;
a) preparing in/or introducing a molten high carbon ferrous metal (hot metal) into an EAF, said molten high carbon ferrous metal having a carbon content of less than 3% and a silicon and manganese content of less than 0.2% each, and a temperature of 1375xc2x0 C. or high in the furnace;
b) adding a solid metallic charge selected from scrap, HBI, DRI (sponge, pellet, etc.) or iron ore present singularly or in any combination together with lime into the hot metal inside the EAF;
c) melting-cum-refining of the entire EAF charge mix in the absence of gaseous oxygen for carrying out said step of melting-cum-refining;
d) superheating, finishing and tapping the molten steel into the ladle, for its subsequent casting.